1. Field of the -Invention
The present invention relates to improved nickel base alloys which have a particularly high combination of strength properties and ductility over the temperature range extending from about room temperature to approximately 1500.degree. F. The improvement is provided by incorporating a substantial amount of tantalum in the alloy, generally as an atom-for-atom replacement for niobium, and then heat treating the alloy at very high temperatures for extended periods.
2. Description of the Prior Art
Prior art nickel-base superalloys while steadily being improved, have disadvantages either from a standpoint of strength or ductility, particularly at elevated temperatures, i.e., above about 1200.degree. F. These alloys are generally based upon nickel in combination with one or more of chromium, iron, and cobalt. In addition, they may contain a variety of elements in a large number of combinations to produce desired effects. Some of the elements which have been utilized in nickel-base superalloys to provide or improve one or more of the following properties are: strength (Mo, Ta, W, Re), oxidation resistance (Cr, A1), phase stability (Ni) or increased volume fractions of favorable secondary precipitates (Co). Other elements are added to form hardening precipitates such as gamma prime (Al, Ti) and gamma double prime (Cb). Minor elements (C, B) are added to form carbides and borides and others (Ce, Mg) are added for purposes of tramp element control. Some elements (B, Zr, Hf) also are added to promote favorable grain boundary effects. Many elements (e.g., Co, Mo, W, Cr), although added for their favorable alloying qualities, can participate, in some circumstances, in the formation of undesirable phases (e.g., sigma, mu, Laves).
Gamma double prime is generally considered to be a body centered tetragonal ordered Ni.sub.3 Nb strengthening precipitate which is formed when niobium is present in nickel-base superalloys. A superalloy in which gamma double prime strengthening occurs is Inconel 718 which is within the scope of U.S. Pat. No. 3,046,108 (Eiselstein). Eiselstein teaches that the alloy must contain about 4 to about 8 weight percent columbium and that the columbium in the alloy may be replaced in part with tantalum in an amount of up to 4% of the alloy. In partially replacing the columbium content of the alloy with tantalum, Eiselstein teaches that double the weight of tantalum should be used to obtain the same effect on properties. He also teaches that only tantalum-free alloys and/or alloys wherein not more than 50% of the columbium is replaced by tantalum are notch-ductile at elevated temperatures. Eiselstein thus teaches that tantalum and niobium act the same in nickel-based alloys provided that only a limited amount of tantalum is present.
The gamma double prime phase is not normally a stable phase since it can convert to gamma prime or to delta on extended exposure to elevated temperatures. Alloys hardened with gamma double prime achieve high tensile strength and very good creep rupture properties at lower temperatures, but the conversion of gamma double prime to gamma prime or delta above about 1250.degree. F. causes a sharp reduction of strength. (Donachie, M. J., "Relationship of Properties to Microstructure in Superalloys" in Superalloys Source Book, American Society for Metals, 1984).